With the rapid development of communication technology, an increasing amount of image information relies on network transmission. While this enhances transmission efficiency, it also raises the risk of illegal attacks. Therefore, this paper proposes a novel multi-image encryption algorithm based on a new spatiotemporal chaos system and fractal geometry. The algorithm begins by designing a new Chebyshev Improved Coupled Sine Map Lattice (CICSML) system based on the Coupled Map Lattice (CML) spatiotemporal chaos system. Dynamical behavior testing results indicate that, compared to traditional chaos systems, the CICSML system can perform chaotic iterations in both time and space, generating a larger number of chaotic sequences, possessing a broader parameter space, and featuring a wider chaotic region. Secondly, the algorithm uses fractal matrix and Hilbert curve scan scrambling to generate index control sequences for the synchronous scrambling diffusion phase. The initial value sensitivity of fractal matrix and the efficiency of Hilbert curve scan scrambling effectively enhance the security and efficiency of the encryption algorithm. Finally, the algorithm employs the principle of multi-image encryption based on indexed image transformation to encrypt multiple images simultaneously. Experimental simulations and performance analysis results show that the algorithm has good security and strong resistance to attacks.

Based on Shil'nikov criterion, i.e., extending the number of unstable saddle-focus-type equilibria of chaotic system, a rich and varied multiwing chaos with complicated topology and remarkable pseudorandomness has been coined. However, it is a challenging task to construct multiwing chaos with only stable node-focus-type equilibria as the Shil'nikov criterion is not applicable in the circumstances. To eliminate this difficulty and enrich the multiwing chaos, we construct a simple three-dimensional autonomous system with only two symmetric stable equilibria, and further introduce a sawtooth wave modulation function to configure more equilibria. The highlight is that multiwing chaotic attractors can be generated from system with two or multiple stable node-foci. This characteristic indicates that the attractors can be hidden oscillating and are difficult to be located in the small neighborhoods of equilibria, which makes the system has good concealment performance for security applications. The design mechanism and system properties are theoretically analyzed and numerically investigated using phase portraits, finite-time Lyapunov exponents, and basins of attraction. Hardware experiments based on field-programmable gate array verify the feasibility of the system. Finally, based on the generated multiwing hidden chaotic attractors, an image encryption scheme with the high security performance is designed to enhance its application.

Due to complex chaotic characteristics and multistable states, the generation of multi-wing hidden chaotic attractors shows a new application prospect in chaos-based security communication. However, for chaotic systems with multiple stable equilibria, the point attractors have attraction basins that are separately embedded in the initial value space, which makes it difficult to generating bands to connect different wings or scrolls. Up to now, grid-wing hidden chaotic attractors with only stable equilibria have not been reported yet. In this paper, a novel system for generating symmetric and asymmetric hidden chaotic attractors is proposed. By applying symmetric piecewise-linear modulation functions to a seed system with only two stable equilibria, multiple stable equilibria with single-direction pair distribution and two-direction grid distribution are configured. Principle of the key technique is based on the offset switch, which effectively connect different wings via the switch bands. Consequently, multi-wing, multi-double-wing, and grid-wing hidden chaotic attractors are generated. Besides, applying asymmetric modulation functions lead to the generation of asymmetric multi-wing hidden chaotic attractor. Attraction basins of various multi-wing and grid-wing hidden attractors are investigated to show the multistability. Numerical simulations and corresponding FPGA-based physical experiments validated the feasibility of the generation method.

In this brief a simple three-dimensional sine chaotic system formed by six terms is presented. The only one nonlinear term, i.e., the sinusoidal function makes the system have infinitely many equilibria. This prominent feature lead to infinitely many coexisting self-excited and hidden attractors with the same or different shapes, also namely homogenous multistability or heterogenous multistability. Furthermore, the multiple equilibria make the system successfully generate multi-scroll attractors. These complicated dynamical behaviors are numerically studied and further experimentally demonstrated by microcontroller-based hardware platform.